It is a well-known fact that the price of AC motors is about half that of commutator DC motors with similar power, but at the same time such AC motors are more reliable and have a longer useful life than DC motors.
An advantage of DC machines, compared to AC motors, is that their speed and torque can be controlled over a wide range; they can be operated not only as motors but also as generators, in which case they apply a braking torque to the driving shafts and supply energy, which is equivalent to the driving shafts and supply energy, which is equivalent to the braking power, back to the current-supply network.
For the most part, thyristor rectifiers are used nowadays to supply DC machines. The output voltage of the thyristor rectifiers increases at the moment of firing. As a result there is a pulsating alternating voltage superimposed on the direct voltage.
This alternating component of the supply voltage increases considerably the load of commutator DC machines. Thus, they must be made more robust (i.e. overdimensioned) in order to prevent damage due to these effects.
The above-mentioned advantages of AC motors cannot be utilized when continuous speed and power regulation is necessary.
Variable three-phase output voltage and frequency from static frequency changers are already being used to drive electrical AC motors.
A similar approach is described by Sandor Marti and Dr. Laszlo Nagy in "The Optimization of I.F. Thyristor Frequency Changers to Supply Grinding-Wheel Motors" (Villamossaq Vol. 21, No. 7, pp. 216-218, July 1973). According to this article, the variable frequency and amplitude of the three-phase output signal is produced in two stages. First, a variable direct voltage is generated by a rectifier controlled by the three-phase network, and then this direct voltage is converted into alternating voltage of the required frequency by using a three-phase inverter. The amplitude of the output voltage is controlled by the direct voltage fed to the inverter, and the frequency is controlled by changing the recurrence rate of the firing pulses supplied to its thyristors.
This approach is basically similar to the inverter of Brown Boveri and CIE (BBC) named VERITRON. Its description can be found in their catalogue D GHS 309320.
Owing to the non-sinusoidal character of the alternating voltage produced by said inverters, the pace of electric motors driven thereby is rough, especially at low frequencies. As a result of the non-sinusoidal supply, the noise level of the motors increases and their running is unfavorable from all points of view in comparison with a motor supplied with a sinusoidal current. During each switching cycle, the thyristors or controlled rectifiers become abruptly conductive so that a filter element should be installed between the inverter and the load to smoothen the output current.
The controlled rectifiers operate on the principle of phase-splitting. Therefore, each network phase conducts only in certain periods of a cycle whose width determines the rectified mean power and, after filtering, the magnitude of the direct voltage available.
The pulsating loading of the three-phase network requires a phase-compensating stage to be inserted between the network and the inverter to protect the network. The costs and sizes of the phase-compensating stage and the filter element are comparable to those of the rest of the frequency changer, especially for high-power units.
In another inverter of Brown Boveri and CIE, described in their catalogue G3A 6019, the thyristors are not switched in the cadence of the output frequency but in the cadence of clock pulses, the frequency of the latter being an integral multiple of the frequency of the output signal. The frequency of the clock pulses is fixed relative to the frequency of the output. A substantially sinusoidal output is produced by comparing the triangular clock pulses to a suitable direct voltage. As a result of this comparison there are generated in each phase a certain number of control pulses during each semiperiod of the basic frequency. The average energy of the control pulses of different width, separated by intervals of de-energization, approximates the energy distribution of the sinusoidal load current. In the control system described in the catalogue mentioned above, the semiperiods of the quasi-sinusoidal output signal encompass three pulses which appears incompatible with satisfactory and steady operation of a motor. Such a system also limits the frequency of the output signal to certain submultiples of the basic clock frequency.
A more elaborate inverter, in which each phase of a three-phase motor is energized by several thyristors in parallel whose conduction is timed by relatively staggered triangular pulse trains, has been described in an article by A. Schonung and H. Stemmler published in BROWN BOVERI MITTEILUNGEN of August/September 1964, Vo. 51, pages 555-577.
Different thyristor-control means for industrial motors are summarized in the catalogue of ASEA of Sweden No. 8297 Ta.
U.S. Pat. No. 3,935,528 describes an inverter with a commutation circuit which comprises a series-resonant network, including a commutating capacitor and an inductance, connected to the midpoint of a balanced DC supply whose positive and negative terminals feed a load via respective circuit branches each including a main thyristor in antiparallel relationship with a respective diode. Two antiparallel ancillary thyristors are inserted between a junction of these branch circuits and the series-resonant network. The two main thyristors conduct in respective intervals of a clock cycle or recurrence period, these intervals being spaced apart sufficiently to prevent simultaneous conduction which would short-circuit the supply. The conduction interval of either main thyristor is terminated by the firing of a respective ancillary thyristor, lying in bucking relationship therewith, which remains conductive only during part of the conduction interval of the other main thyristor. As long as one of the ancillary thyristors conducts, the commutating capacitor reverses its charge via the conducting main thyristor of one circuit branch or the diode of the other branch.